Putting the texture on the object / Displace Modifier
On top view UV select all of the faces  of your road object and do unwrap form view, so that the mapping is even and has the same scale.

Another thing worth trying is not using UVs for mapping on the modifier but an Empty.

Re-sizing and moving the empty might give you more control on the placement of the texture.



To make the displace modifier work smoother you might need to add a subsurf modifier before the displace modifier.

If you are not going to see the pavement texture very close on your image, you might want to consider using it as a bump map, instead of a real 3D displacement, that would require much denser geometry.